The present invention is directed to resonant energy conversion and/or inversion circuits. More particularly, DC-to-DC or AC/DC-to-DC/AC step-up voltage circuits with high efficiency.
DC-DC converters are widely used in step-up ac motor drives, regulated switch-mode DC power supplies, inverters, and DC-motor drives. Often the input to these DC-DC converters is an unregulated DC voltage, which is obtained by rectifying a line voltage. Thus, the DC input voltage normally fluctuates due to changes in the line-voltage magnitude. Switch mode DC-to-DC converters are used to convert the unregulated DC input voltage into a controlled DC output voltage at a desired voltage level. The converters are often used in an electrical isolation transformer in switch mode DC power supplies, and almost always without an isolation transformer in the case of DC-motor drives.
In DC-DC converters an average DC output voltage must be controlled for it to equal a desired level, though an input voltage and an output load impedance may fluctuate. Switch mode DC-DC converters utilize one or more switches to transform DC voltage from one voltage level to another. The average output voltage in a DC-DC converter with a given input voltage is controlled by controlling an on/off duration of a switch, where this average value of output voltage depends on the on-duration and off-duration of the switching signal.
One topology for controlling the average output voltage utilizes switching at a constant frequency and adjusting an on-duration of the switch. When using this topology, called Pulse Width Modulation (PWM) switching, a switch duty ratio D (defined as a ratio of constant switching frequency, the switch control signal, which controls the state (on or off) of the switch, is generated in one of two ways: 1) deriving the PWM signal directly through a known calculation in a microprocessor or 2) by comparing a signal level control voltage with a repetitive waveform. The control voltage signal is generally obtained by amplifying an error value, which is the difference between an actual output voltage and its desired value. The frequency of the repetitive waveform with a constant peak, e.g., a sawtooth or square wave, establishes the switching frequency. This frequency is kept constant in PWM control and is chosen to be in a few kilohertz to a few hundred kilohertz range. When the amplified error signal, which varies very slowly with time relative to the switching signal, is greater than the waveform being used the switch control signal becomes high, causing the switch to turn on. Otherwise, the switch is off.
Step-up converters are used in regulated DC power supplies and regenerative braking of DC motors, where the output voltage is always greater than the input voltage. When a switch is in an ON position, a diode in an input stage is reversed biased, thus isolating an output stage. The input stage is used to supply energy to an inductor. When the switch is in an OFF position, the output stage receives energy from the inductor as well as from the input stage. In steady state, an output filter capacitance theoretically becomes very large, which ensures a constant output voltage. The step-up converter transfers energy in only one direction, which is a direct consequence of it being able to produce only unidirectional voltage and current.
A push-pull inverter requires a transformer with a center-tapped primary. This type of inverter can operate in a PWM or a square-wave mode. The main advantage of the push-pull inverter is that no more than one switch in series conducts at any instant of time. This is important if the DC input to the inverter is from a low-voltage source, such as a battery, where the voltage drop across more than one switch in series would result in a significant reduction in energy efficiency. Feedback diodes connected anti-parallel to the switches are required to carry the reactive current, where their conduction interval depends inversely on the power factor of an output load. These feedback diodes are needed to provide a path for the high current required due to leakage flux of the transformer. In this configuration, there is a slight difference in the switching times of two switches. Thus, there is always an imbalance between the peak values of the two switch currents. This can be controlled through current-mode control of the inverter.
Generally, in a converter with electrical isolation there is a primary and secondary side coupled by a transformer. In one conventional configuration, shown in FIG. 1, the secondary side has a full bridge parallel-loaded resonant (PLR) section, which includes diodes 24, 26, 32, and 34 and a filter section. In operation, assuming the transformer is ideal, when a switch is switched to an ON position on the primary side, diodes 24 and 32 are forward biased and diodes 26 and 32 reverse biased. Then, when the switch is in a switched to an OFF position, the inductor current circulates through diodes 32 and 34, which causes the inductor current to decrease linearly.
Traditionally, there are three configurations of resonant-switch converters, which are alternative devices used in place of the switch-mode controllable switches. First, there is a zero-current switching (ZCS) topology where the switch turns on and off at zero current. The peak resonant current flows through the switch, but the peak switch voltage remains the same as in the switch-mode counterpart. Second, there is a zero-voltage-switching (ZVS) topology where the switch turns on and off at zero voltage. The peak resonant voltage appears across the switch, but the peak switch current remains the same as in the switch-mode counterpart. Third, there is a zero-voltage-switching, clamped-voltage (ZVS-CV) topology where the switch turns on and off at zero voltage. However, a converter of this topology consists of at least one converter leg made up of two such switches. In this third topology, the peak switch voltage remains the same as in its switch-mode counterpart, but the peak switch current is generally higher.
Usually, to soft switch a switch-mode converter, a commutation circuit is needed to turn off the switching device. These commutation circuits circulate a current through a conducting switching device in a reverse direction, and thus force a total switching device current to go to zero, which turns the switch off. These circuits often consist of some form of an L-C resonant circuit driven by a frequency of commutation.
Unfortunately, the efficiency of these above-mentioned conventional circuits is rather low, e.g., 80%. Also, it is usually quite complex a design, e.g., parallel connection of low-power converter, if one wants to increase the efficiency in order to achieve Vout/Vinxe2x89xa710 with Powerxe2x89xa72 kW. Therefore, what is needed is a resonance conversion circuit that has a simple circuit topology with high efficiency, which can achieve the above input/output voltage ratio and power parameters desired.
This present invention overcomes all these above-mentioned shortcomings of the prior art devices through use of a resonance section in a DC-DC converter circuit that is operatively configured to produce multiple voltage, i.e., the Alexander topology circuit.
The present invention includes a circuit comprising a primary section and a multiple voltage secondary section. The multiple voltage secondary section includes a multiple voltage resonance section, a filter section, and a load coupling section. The circuit also includes a transformer, which is operatively configured to couple the primary and secondary sections. The multiple voltage resonance section includes an equivalent capacitance, i.e., the Alexander topology circuit. This equivalent capacitance, which is preferably two series capacitors, operatively generates a predetermined voltage, such that a turns ratio of the transformer is lower than a non-multiple voltage resonance section to generate the predetermined voltage. Hence, this configuration decreases energy loss from the transformer, thereby increasing efficiency.
An advantage of the present invention is that by using this topology a circuit with a simple configuration operates at a high efficiency, i.e., above 90%.
Another advantage of the present invention is that through the use of multi-voltage energy conversion, a turns ratio of a transformer is half as big compared to conventional topologies, while the input and output energy parameters remain the same. Hence, this topology produces higher efficiency.
A still further advantage of the present invention is that since the voltage on reactive components is lower than in the conventional typologies, the size of these components can be reduced. Thus, the overall circuit is smaller and costs less.
Another still further advantage of the present invention is that by decreasing the variable at the input of the output filter, this topology allows the reduction of reference power of the output filter. In alternative embodiments an output filter can be dispensed with altogether.